I know that when I want to calculate Depth of Field by hand one of the variable elements in that equation is the Circle of Confusion. In layman's terms, what is the "Circle of Confusion," how do I calculate it, and is there any other ways that it applies to my photography aside from calculating DoF?

I know you've gotten suspicious looks for asking questions before ;) but it is great to see someone with so much experience still willing ask questions and learn. First rate people always want to learn more.
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ItaiFeb 22 '11 at 15:08

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@Itai: Thanks! Most of the time I actually do know the answers to the questions I ask, but in this case my understanding of CoC is fuzzy at best... :-)
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Jay Lance PhotographyFeb 22 '11 at 22:46

2 Answers
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This is often a source of confusion which most people get backwards, so understanding this is delicate:

When a light entering a lens is not in focus, a point on the subject is focused into a circle on the image plane (sensor/film). This circle IS the circle of confusion. The more out of focus a point is, the larger the circle of confusion becomes. This depends on focus distance, subject distance and aperture. It does not depend on the capture device resolution or viewing conditions.

The circle of confusion used in DOF calculations is the maximum allowable circle of confusion which is considered in acceptable focus. This is dictated by the medium size and viewing distance because of the way human vision resolves details.

Historically, most DOF tables use a standard COC which corresponds to unaided viewing of an 8"x10" at 14" away for someone with 20/20 vision, although I am sure other magic numbers are used sometimes.

One minor proviso: in a lens that has any coma (which is all of them, of course), the circle may be rather (or even quite) non-circular toward the corners of the picture, especially if (or example) produced by a high-contrast point of light.
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Jerry CoffinFeb 20 '11 at 5:50

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Another "point" to remember is that lenses can't focus perfectly, so even at the plane of focus a point is resolved as a dot and the size of that dot is known as the circle of least confusion. This puts a kind of a limit on the useful resolving power of both film and digital sensors.
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David RouseFeb 20 '11 at 18:01

@Jerry - Yes, I know. Just keeping things simple for the sake of shortness as it does not affect DOF.
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ItaiFeb 20 '11 at 20:38

a lens is actually in focus at one point. as you move further from that point, the image blurs more. The amount of blur at a given distance from the focal point depends on focal length of the lens and the distance to the focal point.

The circle of confusion is the largest area blur that is indistinguishable from a point. It's dependent upon sensor resolution and film acuity. It also depends on viewing conditions and print size. The numbers you see used in calculations are just accepted standards.

It tends to be an easier concept for large format photogs as they deal with focal planes more.